1. Field of the Invention
This invention relates to improvements in laser gyroscopes, and more particularly to circuits for increasing the resolution of a laser gyroscope.
2. Description of the Prior Art
Laser gyroscopes are known in the art to have counter-rotating light beams in a cavity, the counter-rotating beams being sensed through partially transmissive mirrors by combining optics to produce spaced-apart elongated patterns of light on an array of spaced-apart elongated photo detectors. The elongated patterns of light are referred to as fringe patterns. The fringe patterns remain stationary on the photo detector array while the frequencies of the counter rotating light beams are identical. Rotation of the gyroscope body on its sensitive axis normal to the plane of the light beams results in counter-propagating light beams of different frequencies. The fringe patterns translate across the photo detector array with increasing frequency in response to an increasing input body rate and they translate with decreasing frequency in response to a decreasing input body rate. A clockwise input body rate produces apparent fringe pattern motion in a first direction and a counterclockwise input body rate produces apparent fringe pattern motion across the photo diode detector array in an opposite direction.
The laser gyroscope readout is performed by biasing the photo diode detectors to produce an electrical signal having an amplitude proportional to the incident light intensity. The signal from each diode varies sinusoidally in response to movement of the fringe patterns across the photo diode detector array. The photo diode detectors are spaced and positioned to provide at least two quadrature signals (i.e. sinusoidal signals separated in phase by essentially ninety degrees) in response to movement of the fringe patterns across the photo diode detector array.
The quadrature signals obtained from the photo diode detector array are amplified and squared to provide a pair of digital quadrature signals having a first and second logic state. Electronic quadrature decoders sense the digital quadrature signals and provide a digital mode signal indicating the direction of gyro rotation, and count signals indicating the frequency of gyro rotation to a system of counters for integration to provide a continuous readout of the gyroscope body rotational angle.
Electronic quadrature decoder circuits can increase the resolution of a laser gyroscope by a factor of two or four through simple decoding at half-cycle (zero crossings) of each amplified sinusoidal signal from the photo detector array. To increase the resolution requires a scheme of either frequency or phase multiplication of the amplified quadrature signals; thereby, increasing the frequency for phase resolution. However, multiplying techniques are limited in resolution to the basic phase errors inherent in the amplified quadrature signals. One source of phase error is that error due to tolerance buildup associated with assembly of the combining optics and photo detector array. This phase error contributes to an increase in the spatial fringe pattern quadrature angle error. The reduction of this quadrature error to limits below that inherent to the gyro allows the use of increased phase resolution circuits in the readout decoder. Prior art laser gyroscopes have limited this error by tightening the mechanical and assembly tolerances to acceptable limits. Tighter mechanical and assembly tolerances contribute to higher instrument cost, and serves to limit the resolution achievable.